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Abstract

Single crystals of the title compound, NaTb(PO3)4, were obtained by solid-state reaction. This compound belongs to type II of long-chain polyphosphates with the general formula AIBIII(PO3)4. It is isotypic with the NaNd(PO3)4 and NaEr(PO3)4 homologues. The crystal structure is built up of infinite crenelated chains of corner-sharing PO4 tetra­hedra with a repeating unit of four tetra­hedra. These chains, extending parallel to [100], are linked by isolated TbO8 square anti­prisms, forming a three-dimensional framework. The Na+ ions are located in channels running along [010] and are surrounded by six oxygen atoms in a distorted octa­hedral environment within a cut-off distance <2.9 Å.

supplementary crystallographic
information

Comment

It is now well established that long-chain polyphosphates with general formula
AIBIII(PO3)4 can be divided in seven structural types
(Jaoudi et al., 2003). All long-chain polyphosphates of formula
NaLnP4O12 (Ln = rare earth element) reported up to now ( El
Masloumi et al. 2005; Zhu et al. 2006; Zhu et
al. 2008;
Horchani-Naifer et al. 2009; Koizumi et al. 1976;
Amami et
al. 2005; El Masloumi et al. 2008; Amami et
al. 2004;
Maksimova et al. 1988) belong to the structural type II which
has been
first described on basis of the KCe(PO3)4 structure (Linde et al.,
1983). A few other AIBIII cationic combinations such
as AgGd
(Naïli et al. 2006) and NaBi (Jaoudi et al. 2003; Belam et
al. 2007) also lead to polyphosphates which belong to the
structural type
II. The structure of the title compound also fits in this isotypic series. The
underlying structure has many times been described as built up of
(PO3)∞ chains running along the [100] direction and further linked
by isolated LnO8 polyhedra. The resulting three dimensional framework
delimits tunnels where the Na+ ions are located. Instead of using this
description, we will focus on the connectivity between the (PO3)∞
chains and the TbO8 square antiprisms for our account. Each TbO8 square
antiprism is linked to four (PO3)∞ chains by corner-sharing
involving the non-bridging oxygen atoms of the PO4 groups that exhibit the
shorter P—O distances within the chain. Their P—O distances range from
1.4792 (13) Å to 1.4918 (13) Å. The chains are crenelated with a repeating
unit of four corner-sharing tetrahedra, as displayed in Fig. 1. The repeating
unit is built up of PO4 tetrahedra corresponding to the four
crystallographically independent phosphorus atoms labelled from P1 to P4. If
the origin of the chain is taken at the O2 position for instance, then the P2
and P4 tetrahedra are the end-groupings of the repeating unit while P1 and P3
tetrahedra are involved in the internal diphosphate group. Each
(PO3)∞ chain is linked to four rows of isolated TbO8 square
antiprisms parallel to the direction of the chain (Fig. 2). With the
aforementioned origin convention both terminal P(2)O4 and P(4)O4
tetrahedra are connected in a bidentate fashion on one side of the square face
of the archimedean antiprisms of the first row while the internal
P(1)O4—P(3)O4 diphosphate group is also connected in a bidentate fashion
on one side of the square face of the antiprisms of the second row (Fig. 3a
and 3 b). Therefore the two rows of TbO8 polyhedra are translated with a
half-period of the (PO3)∞ chain relative to one another (Fig. 3c).
Thus the tetrahedra involved in the internal P2O7 groups share their
non-bridging oxygen atoms with two TbO8 polyhedra belonging to each of the
first and second rows, respectively. Then the (PO3)∞ chain is
connected to the third and fourth rows in a similar way but the role played by
the couples P(1)O4—P(3)O4 and P(2)O4—P(4)O4 are inverted, this
last becoming the internal diphosphate group.

For a general review on the crystal chemistry of polyphosphates, see:
Durif (1995).

Experimental

Crystals of the title compound were synthesized by reacting Tb4O7 with
(NH4)H2PO4 and Na2CO3 in a platinum crucible. A mixture of these
reagents in the molar ratio 5 : 85 : 10 was used for the synthesis. The
mixture has first been heated at 473 K for 12 h, then at 573 K for 12 h and
finally at 773 K for 24 h. The muffle furnace was then cooled down first to
723 K at the rate of 2 K.h-1 and then to room temperature at the rate of
15 K.h-1. Single crystals were extracted from the batch by washing with
hot water.

Refinement

The highest residual peak in the final difference Fourier map was located 0.61 Å from atom Tb and the deepest hole was located 0.45 Å from atom Tb.

Details of the connections between the (PO3)∞ chains and the TbO8 polyhedra: a) view showing the two kind of bidentate attachments. b) view showing the bidentate attachments and the PO4 groups shared between two TbO8 square antiprisms belonging...

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are
estimated using the full covariance matrix. The cell esds are taken into
account individually in the estimation of esds in distances, angles and
torsion angles; correlations between esds in cell parameters are only used
when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc. and is
not relevant to the choice of reflections for refinement. R-factors
based on F2 are statistically about twice as large as those based on
F, and R- factors based on ALL data will be even larger.